redearslider.com

Fatty (I'm going to call him that till he starts eating something else) will only eat mealworms, and occasionally a few bites of store kind shrimp
Any suggestions?

Is he a hatchling? If he isn't going to eat then just put in pellets or veggies if he's a adult, he'll come around. Mealworms and shrimp as a diet isn't healthy. If he's a hatchling then you'll need to feed him pellets daily.

He's an adult. So I just throw in pellets and he'll eat them eventually?

Remember, you may also want to give the turtle privacy during feeding time.
You can go hide somewhere where you can see the turtle but the turtle can't see
you. Don't walk around though, turtle senses vibration really well.

Tough love approach... mealworms and shrimp are not a healthy diet. Don't give it to him... start with veggies and only offer those for a couple of days.

Like steve said, stop offering mealworms and shrimp. He will eventually be hungry and will eat pellets and veggies. Be sure to leave veggies in his tank daily and soon you'll see them disappear.